With polyolefins, such as polyethylene, polypropylene, polybutane-1, ethylene-propylene rubber, the oxygen index specified in the Japanese Industrial Standards JIS K 7201-1976 entitled "Combustion test method of polymers by the oxygen index method" ranges between 17 and 19. Among the plastics, the above-listed polyolefins are most inflammable and regarded as difficult to be rendered highly flame-retardant. Particularly polyethylene, a general-purpose resin, has excellent properties except combustion characteristic and is most inexpensive synthetic polymer. Therefore, a strong demand is made to render this polymer flame-retardant. Accordingly, development is strongly pushed for a process of ensuring the uninflammability of polyethylene. The conventional process of rendering polyolefins flame-retardant comprises: (1) addition of halogen compounds, (2) addition of halogen compounds+antimony trioxide, (3) addition of halogen compounds+phosphorus compounds. It is considered indispensable to apply halogens such as chlorine or bromine in order to render polyolefins flame-retardant. However, addition of a halogen series flame-retardant is indeed prominently useful to ensure the flame-retardancy of polyolefins, but is accompanied with the drawbacks that when combusted, the polyolefins noticeably evolve black smokes and corrosive and noxious gases including carbon monoxide and gases released from halogen-bearing compounds. In recent years, therefore, it is very much desired to develop a process of ensuring the flame retardancy of polyolefins. Therefore, various processes have been proposed which comprise addition of a special smoke retardant such as molybdenum trioxide to polyolefins or mixing of polyolefins with an inorganic material. To date, however, no practically acceptable process has been established. Among these proposed processes, mixing of polyolefins with an inorganic material attracts attention as a most useful means from the standpoint of saving natural resorces, facilitating incineration and eliminating environmental polutions as recently demanded from other circles of society.
For example, Japanese Patent Publication No. 51-34866 sets forth a flame-retardant composition containing ethylene-vinyl acetate copolymer, silane and hydrated inorganic filler. This composition is intended for insulation of building wiring, implement wiring and automobile wiring and undergoes cross-linking at a very high rate of 95% by irradiation of high energy electron application of a chemical cross-linking agent. The above-mentioned Japanese Patent Publication No. 51-34866 simply describes that a small amount of any other cross-linking polymer, though harmful, may be added to the aforesaid ethylene-vinyl acetate copolymer requiring addition of a silane compound, but such a polymer blend is not preferred for said invention.
Japanese Patent Disclosure (Kokai) No. 49-78741 discloses that it is possible to obtain an olefinic resin composition having excellent heat-collecting and heat-insulating properties by adding alumina trihydrate and metal oxide to a blend polymer consisting of ethylene-vinyl acetate copolymer and polyolefin resin. This invention is not characterized by applying a special type of polyethylene, but is characterized in that it is indispensable to add a metal oxide to ensure the effect of a heat-collecting property. With this invention, a filler of alumina trihydrate and metal oxide are applied in as large a total amount as over 1,000 phr (parts per hundred) of resin. The ethylene-vinyl acetate copolymer is added to elevate the adhesivity of the product to, for example, an aluminium board.
Japanese Patent Disclosure (Kokai) No. 50-145448 and Japanese Patent Disclosure (Kokai) No. 52-12134 respectively refer to a filler and composition containing a titanate series compound. These inventions describe that one of the favorable effects resulting from addition of a titanate series compound is to increase the elongation at break of various polymers. However, the properties set forth in said inventions are still unsatisfactory for the object of the present invention. In other words, the disclosed inventions simply refer to the effect of the high extensibility of the product.